Negative electrode for power storage device and power storage device

ABSTRACT

A decomposition reaction of an electrolyte solution and the like caused as a side reaction of charge and discharge is minimized in repeated charge and discharge of a lithium ion battery or a lithium ion capacitor, and thus the lithium ion battery or the lithium ion capacitor can have long-term cycle performance. A negative electrode for a power storage device includes a negative electrode current collector and a negative electrode active material layer which includes a plurality of particles of a negative electrode active material. Each of the particles of the negative electrode active material has an inorganic compound film containing a first inorganic compound on part of its surface. The negative electrode active material layer has a film in contact with an exposed part of the negative electrode active material and part of the inorganic compound film. The film contains an organic compound and a second inorganic compound.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a negative electrode for a powerstorage device and a power storage device.

2. Description of the Related Art

A variety of power storage devices, for example, non-aqueous secondarybatteries such as lithium ion batteries (LIBs), lithium ion capacitors(LICs), and air cells have been actively developed in recent years. Inparticular, demand for lithium ion batteries with high output and highenergy density has rapidly grown with the development of thesemiconductor industry, as in the cases of electronic appliances, forexample, portable information terminals such as mobile phones,smartphones, and laptop computers, portable music players, and digitalcameras; medical equipment; and next-generation clean energy vehiclessuch as hybrid electric vehicles (HEVs), electric vehicles (EVs), andplug-in hybrid electric vehicles (PHEVs). The lithium ion batteries areessential for today's information society as chargeable energy supplysources.

A negative electrode for the power storage devices such as the lithiumion batteries and the lithium ion capacitors is a structure bodyincluding at least a current collector (hereinafter referred to as anegative electrode current collector) and an active material layer(hereinafter referred to as a negative electrode active material layer)provided over a surface of the negative electrode current collector. Thenegative electrode active material layer contains an active material(hereinafter referred to as a negative electrode active material), suchas carbon or silicon, which can store and release lithium ions servingas carrier ions.

At present, a negative electrode of a lithium ion battery using agraphite based carbon material is generally formed by mixing graphite(black lead) that is a negative electrode active material, acetyleneblack (AB) as a conductive additive, PVdF that is a resin as a binder toform slurry, applying the slurry over a current collector, and dryingthe slurry, for example.

Such a negative electrode of a lithium ion battery and a lithium ioncapacitor has an extremely low electrode potential and a high reducingability. For this reason, an electrolyte solution using an organicsolvent is reduced and decomposed. The range of potentials in which theelectrolysis of an electrolyte solution does not occur is referred to asa potential window. Although the negative electrode essentially needs tohave an electrode potential in the potential window of the electrolytesolution, the negative electrode potential of a lithium ion battery or alithium ion capacitor is out of the potential windows of almost allelectrolyte solutions. In actually a decomposition product thereof formsa passivating film (also referred to as a solid electrolyte film (solidelectrolyte interphase)) on the surface of the negative electrode, andthe passivating film prevents further reductive decomposition. Thus,lithium ions can be inserted into the negative electrode with the use ofa low electrode potential below the potential window of the electrolytesolution (for example, see Non-Patent Document 1).

REFERENCE

-   [Non-Patent Document 1] Zempachi Ogumi, “Lithium Secondary Battery”,    Ohmsha, Ltd., first impression of the first edition published on    March 20, H20, pp. 116-118

SUMMARY OF THE INVENTION

The passivating film is a reductive decomposition product of a reductivedecomposition reaction of the electrolyte solution or a product of areaction between the reductive decomposition product and the electrolytesolution. For example, in the case where a negative electrode activematerial is graphite, since the graphite has a layered structure, apassivating film is formed between layers in an edge surface of thegraphite and a surface (basal surface) of the graphite. When carrierions are inserted into the graphite and thus the volume of the graphiteincreases, part of the passivating film is separated from the graphiteand part of the graphite is exposed.

Although generation of the passivating film kinetically suppressesdecomposition of the electrolyte solution, the thickness of thepassivating film gradually increases due to repeated charge anddischarge. The passivating film having an increased thickness issusceptible to the volume expansion of the negative electrode activematerial, and part of the passivating film is easily separated.

Another passivating film is formed on a surface of the negativeelectrode active material which is exposed by the separation of thepassivating film.

A passivating film of a conventional negative electrode has beenconsidered to be formed due to a battery reaction at the time of charge,and electrical charges consumed in the formation of the passivating filmare the cause for irreversible capacity. This results in a decrease incapacity of a lithium ion battery. In addition, separation of thepassivating film due to repeated charge and discharge and formation ofanother passivating film further reduce the capacity of the lithium ionbattery.

Further, the higher the temperature is, the faster the electrochemicalreaction is. Accordingly, the capacity of the lithium ion batterydecreases more significantly, as charge and discharge are repeated athigh temperature.

The above problems exist not only in lithium ion batteries but also inlithium ion capacitors.

In view of the above, an object of one embodiment of the presentinvention is to stabilize a surface of a negative electrode activematerial in a negative electrode for a lithium ion battery or a lithiumion capacitor. Further, another object of one embodiment of the presentinvention is to minimize electrochemical decomposition of an electrolytesolution and the like in the negative electrode.

Further, an object of one embodiment of the present invention is tominimize a decomposition reaction of an electrolyte solution and thelike caused as a side reaction of charge and discharge in repeatedcharge and discharge of a lithium ion battery or a lithium ion capacitorso that the lithium ion battery or the lithium ion capacitor haslong-term cycle performance.

It is probable that decomposition of an electrolyte solution occurselectrochemically. Graphite or silicon is generally used as a negativeelectrode active material, and electric conductivity thereof isrelatively high. Even silicon that is a semiconductor has high electricconductivity in the state where lithium is inserted into the silicon.For this reason, a decomposition reaction of an electrolyte solutiontakes place on a surface of a negative electrode active material.

On the other hand, a particulate negative electrode active material withan average diameter of several hundred nanometers to several tens ofmicrometers is used in a negative electrode of a lithium ion battery ora lithium ion capacitor to maintain a constant speed of charge anddischarge. Thus, the negative electrode can be regarded as a porouselectrode that is an aggregate of particles of the negative electrodeactive material and the surface area thereof is large. Consequently, anarea where a battery reaction can occur is large, which increases theoccurrence of the decomposition reaction of an electrolyte solution.

Hence, a negative electrode active material having an inorganic compoundfilm on part of its surface and a film typified by a passivating film(hereinafter referred to as a film), which is in contact with an exposedportion of the negative electrode active material and the inorganiccompound film, are used to form a negative electrode active materiallayer. As a result, in the inorganic compound film provided on the partof the surface of the negative electrode active material, electronconductivity can be suppressed while the conductivity of carrier ions isensured, which makes it possible to minimize the decomposition reactionof the electrolyte solution on the surface of the negative electrodeactive material.

Furthermore, one embodiment of the present invention is a negativeelectrode for a power storage device including a negative electrodecurrent collector and a negative electrode active material layer whichis over the negative electrode current collector and includes aplurality of particles of a negative electrode active material. Each ofthe particles of the negative electrode active material has an inorganiccompound film containing a first inorganic compound as a component onpart of its surface. The negative electrode active material layer has afilm in contact with an exposed portion of part of the negativeelectrode active material and part of the inorganic compound film. Thefilm contains an organic compound and a second inorganic compound ascomponents.

A particulate material is used for the negative electrode activematerial. For example, a particulate negative electrode active materialwith an average diameter of more than or equal to 6 μm and less than orequal to 30 μm can be used. For a material of the negative electrodeactive material, graphite that is a carbon material generally used inthe field of power storage can be used. Examples of graphite include lowcrystalline carbon such as soft carbon and hard carbon and highcrystalline carbon such as natural graphite, kish graphite, pyrolyticgraphite, mesophase pitch based carbon fiber, meso-carbon microbeads(MCMB), mesophase pitches, petroleum-based coke, and coal-based coke.Further, a material which is alloyed and dealloyed with carrier ionsgiving and receiving electrical charges may be used. Examples of such amaterial include magnesium, calcium, aluminum, silicon, germanium, tin,lead, arsenic, antimony, bismuth, silver, gold, zinc, cadmium, mercury,and the like.

Other than lithium ions used for lithium ion batteries or lithium ioncapacitors, examples of carrier ions include alkali-metal ions such assodium ions and potassium ions; alkaline-earth metal ions such ascalcium ions, strontium ions, and barium ions; beryllium ions; magnesiumions; and the like.

The inorganic compound film provided on part of a surface of theparticulate negative electrode active material contains the firstinorganic compound as a component. As the first inorganic compound, anoxide of any one of niobium, titanium, vanadium, tantalum, tungsten,zirconium, molybdenum, hafnium, chromium, aluminum, and silicon or anoxide containing any one of these elements and lithium can be used. Theinorganic compound film is much denser than a passivating filmconventionally formed on a surface of a negative electrode due to areductive decomposition reaction of an electrolyte solution. Inaddition, since the inorganic compound film is stable in charge anddischarge, the thickness thereof slightly changes due to charge anddischarge or does not change.

Having carrier ion conductivity, the inorganic compound film cantransmit carrier ions, and therefore the battery reaction of thenegative electrode active material can occur. On the other hand, havinglow electron conductivity and an insulating property, the inorganiccompound film can suppress a reaction between the electrolyte solutionand the negative electrode active material. For these reasons, amaterial which has a high carrier ion diffusion coefficient and whoseelectron conductivity is as low as possible is preferably used for theinorganic compound film. Further, since the inorganic compound filmitself does not function as the active material for the batteryreaction, the inorganic compound film is preferably thin enough totransmit carrier ions. The thickness of the inorganic compound film ispreferably more than or equal to 5 nm and less than or equal to 50 nm.

For example, niobium oxide (Nb₂O₅) which can be used for the inorganiccompound film provided on part of the surface of the particulatenegative electrode active material has a low electron conductivity of10⁻⁹ S/cm² and a high insulating property. Thus, a niobium oxide filminhibits an electrochemical decomposition reaction between the negativeelectrode active material and the electrolyte solution. On the otherhand, niobium oxide has a lithium diffusion coefficient of 10⁻⁹ cm²/secand high lithium ion conductivity, and therefore can transmit lithiumions. However, an inorganic compound film with an increased thicknessincreases an insulating property, and lithium ions are prevented fromfreely moving inside and outside the negative electrode active material;thus, a battery reaction cannot occur. Therefore, the thickness of theniobium oxide film is preferably more than or equal to 5 nm and lessthan or equal to 50 nm.

When carrier ions solvated with a solvent of the electrolyte solutionpass through the inorganic compound film provided on part of the surfaceof the negative electrode active material, they are desolvated, and onlythe carrier ions are diffused into the negative electrode activematerial. Therefore, the inorganic compound film provided on part of thesurface of the negative electrode active material reduces an area of aportion where the electrolyte solution is directly in contact with thenegative electrode active material, which makes it possible to suppressa reductive decomposition reaction of the electrolyte solution.Consequently, an increase in the thickness of the film provided over theexposed portion of the negative electrode active material and theinorganic compound film can be suppressed, and separation of the filmdue to repeated charge and discharge can be suppressed.

Note that it is preferable that the negative electrode active materialbe not covered with the inorganic compound film completely and at leastpart of the negative electrode active material be exposed to ensure apath for electronic conduction between the negative electrode activematerial and the outside.

Further, the film provided over the exposed portion of the negativeelectrode active material and the inorganic compound film contains theorganic compound and the second inorganic compound as the components.The organic compound is a reductive decomposition product of thereductive decomposition reaction of the electrolyte solution or aproduct of a reaction between the reductive decomposition product andthe electrolyte solution. The second inorganic compound is a product ofa reaction between the carrier ions and an organic compound contained inthe electrolyte solution and is, typically, one of a fluoride, acarbonate, an oxide, and a hydroxide which include a metal element ofthe carrier ions. Therefore, the second inorganic compound is differentfrom the first inorganic compound.

The film has high carrier ion conductivity and low electronconductivity. For this reason, when the inorganic compound film is notprovided on the negative electrode active material and a surface of thefilm is the surface of the negative electrode active material layer, thereductive decomposition reaction of the electrolyte solution can besuppressed. However, the suppression is temporary, and the thickness ofthe film increases due to repeated charge and discharge and thus part ofthe film is separated.

In one embodiment of the present invention, the film is provided on asurface of the inorganic compound film provided on part of the surfaceof the negative electrode active material and on the exposed portion ofthe negative electrode active material; thus, adhesion between theinorganic compound film and the film can be increased. As a result,separation of the film provided on the exposed portion of the negativeelectrode active material and the surface of the inorganic compound filmdue to repeated charge and discharge can be suppressed.

Further, the inorganic compound film is thinned so that carrier ions canpass through the inorganic compound film and the film is provided incontact with the exposed portion of the negative electrode activematerial and the inorganic compound film, which makes it possible tosuppress the electron conductivity of the inorganic compound film andthe film while the carrier ion conductivity of the inorganic compoundfilm and the film is secured. In other words, it is possible to minimizethe reductive decomposition reaction of the electrolyte solution on thesurface of the negative electrode active material; consequently, adecrease in the capacity of the power storage device can be suppressed.

As described above, the inorganic compound film is formed on part of thesurface of the particulate negative electrode active material and thefilm is provided on the exposed portion of the negative electrode activematerial and the inorganic compound film; thus, the surface of thenegative electrode active material can be stable, and the decompositionreaction of the electrolyte solution can be minimized while the batteryreaction of the negative electrode active material is possible.

According to one embodiment of the present invention, a negativeelectrode active material of a lithium ion battery or a lithium ioncapacitor has a stable surface, which makes is possible to minimizeelectrochemical decomposition of an electrolyte solution and the like ina negative electrode.

Further, according to one embodiment of the present invention, adecomposition reaction of an electrolyte solution and the like caused asa side reaction of charge and discharge can be minimized in repeatedcharge and discharge of a lithium ion battery or a lithium ioncapacitor, and thus the lithium ion battery or the lithium ion capacitorcan have long-term cycle performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D illustrate a negative electrode and a negative electrodeactive material having an inorganic compound film.

FIG. 2 illustrates a method for forming a negative electrode activematerial having an inorganic compound film.

FIGS. 3A to 3D illustrate a negative electrode.

FIGS. 4A to 4C illustrate a positive electrode.

FIGS. 5A and 5B illustrate a coin-type lithium ion battery.

FIG. 6 illustrates a laminated lithium ion battery.

FIGS. 7A and 7B illustrate a cylindrical lithium ion battery.

FIG. 8 illustrates electronic appliances.

FIGS. 9A to 9C illustrate an electronic appliance.

FIGS. 10A and 10B illustrate an electronic appliance.

FIG. 11 shows an X-ray diffraction spectrum.

FIGS. 12A and 12B are SEM images.

FIGS. 13A and 13B are SEM images.

FIG. 14 is a TEM image.

FIGS. 15A and 15B are TEM images.

FIGS. 16A and 16B show results of CV measurement.

FIGS. 17A and 17B show results of CV measurement.

FIG. 18 is a TEM image.

FIGS. 19A to 19C show results of EDX analysis.

FIG. 20 shows cycle performance.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and an example are described below with reference todrawings. However, the embodiments and the example can be implementedwith various modes. It will be readily appreciated by those skilled inthe art that modes and details can be changed in various ways withoutdeparting from the spirit and scope of the present invention. Thus, thepresent invention should not be interpreted as being limited to thefollowing description of the embodiments and the example.

(Embodiment 1)

In this embodiment, a structure of a negative electrode active materialhaving an inorganic compound film which can suppress a decompositionreaction of an electrolyte solution is described with reference to FIGS.1A to 1D.

FIGS. 1A and 1B illustrate a negative electrode for a power storagedevice of the present invention. FIGS. 1C and 1D each illustrate anegative electrode active material used in the negative electrode for apower storage device.

FIG. 1A is a cross-sectional view of a negative electrode 10 including anegative electrode current collector 11 and a negative electrode activematerial layer 13 provided on one or both surfaces (on one surface inFIG. 1A) of the negative electrode current collector 11.

FIG. 1B is an enlarged cross-sectional view of part of the negativeelectrode active material layer 13 surrounded by the dashed line 17 inFIG. 1A. The negative electrode active material layer 13 includes anegative electrode active material 101 having an inorganic compound film102 on part of its surface. In addition, the negative electrode activematerial layer 13 includes a film 103 which is in contact with anexposed portion of part of the negative electrode active material 101and part of the inorganic compound film 102.

The negative electrode active material 101 is a particle, a fineparticle, or powder (hereinafter the negative electrode active materialis also referred to as a particulate negative electrode activematerial). Particles of the negative electrode active material 101 arenot necessarily in a spherical shape and the particles may have givenshapes different from each other. As the particulate negative electrodeactive material 101, a commercial negative electrode active material canbe used. For example, a particulate negative electrode active materialwith an average diameter of more than or equal to 6 μm and less than orequal to 30 μm can be used. A method for forming the negative electrodeactive material 101 is not limited as long as the negative electrodeactive material 101 has the above-described shape.

For the material of the negative electrode active material 101, graphitethat is a carbon material generally used in the field of power storagecan be used. Examples of graphite include low crystalline carbon such assoft carbon and hard carbon and high crystalline carbon such as naturalgraphite, kish graphite, pyrolytic graphite, mesophase pitch basedcarbon fiber, meso-carbon microbeads (MCMB), mesophase pitches,petroleum-based coke, and coal-based coke. Graphite has a low potentialsubstantially equal to that of a lithium metal (0.1 V to 0.3 V vs.Li/Li⁺) when lithium ions are intercalated into the graphite (when alithium-graphite intercalation compound is generated). For this reason,a lithium ion battery can have a high operating voltage. In addition,graphite is preferable because of its advantages such as relatively highcapacity per volume, small volume expansion, low cost, and safetygreater than that of a lithium metal.

Alternatively, a material which is alloyed and dealloyed with carrierions giving and receiving electrical charges may be used. Examples ofsuch a material include magnesium, calcium, aluminum, silicon,germanium, tin, lead, arsenic, antimony, bismuth, silver, gold, zinc,cadmium, mercury, and the like. Such elements have higher capacity thancarbon. In particular, silicon has a theoretical capacity of 4200 mAh/g,which is significantly high. For this reason, silicon is preferably usedas the negative electrode active material. Examples of an alloy-basedmaterial using such elements include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like.

Further alternatively, as the negative electrode active material,lithium titanium oxide (Li₄Ti₅O₁₂), a lithium-graphite intercalationcompound (Li_(x)C₆), or the like can be used.

Still further alternatively, as the negative electrode active material,Li_(3−x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N is preferable because of high capacity (900mAh/g).

The nitride containing lithium and a transition metal is preferablyused, in which case lithium ions are contained in the negative electrodeactive material, and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈.Note that even in the case of using a material containing lithium ionsas the positive electrode active material, the nitride containinglithium and a transition metal can be used as the negative electrodeactive material by extracting lithium ions contained in the positiveelectrode active material in advance.

Still further alternatively, as the negative electrode active material,a material which causes a conversion reaction can be used. For example,a transition metal oxide which does not cause an alloying reaction withlithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide(FeO), may be used. Other examples of the material which causes aconversion reaction include oxides such as Fe₂O₃, CuO, Cu₂O, and RuO₂,sulfides such as CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N,and Ge₃N₄, phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides suchas FeF₃ and BiF₃. Note that any of the fluorides can be used as apositive electrode active material because of its high potential.

The inorganic compound film 102 is provided on a surface of a particleof such a negative electrode active material 101. As illustrated in FIG.1C, the inorganic compound film 102 does not cover the surface of theparticle of the negative electrode active material 101 entirely, andcover the surface partly. Thus, there are a region covered with theinorganic compound film 102 and a region not covered with the inorganiccompound film 102 in the surface of the particle of the negativeelectrode active material 101. In addition, the inorganic compound film102 covering the particle of the negative electrode active material 101may have a relatively large surface covering a few percent to severaltens of percent of the surface area of the particle of the negativeelectrode active material 101 as illustrated in FIG. 1C or a surfacewith a very small area as illustrated in FIG. 1D. The size of theinorganic compound film 102 attached on the surface of the particle ofthe negative electrode active material 101 can be appropriately adjusteddepending on conditions of a sol-gel method described later, the shapeor state of a surface of a negative electrode active material to beused, or the like.

As the material of the inorganic compound film 102, an oxide film of anyone of niobium, titanium, vanadium, tantalum, tungsten, zirconium,molybdenum, hafnium, chromium, aluminum, and silicon or an oxide filmcontaining any one of these elements and lithium can be used. Theinorganic compound film 102 formed using such a material is much denserthan a passivating film conventionally formed on a surface of a negativeelectrode active material due to a decomposition product of anelectrolyte solution.

For example, niobium oxide (Nb₂O₅) has a low electron conductivity of10⁻⁹ S/cm² and a high insulating property. For this reason, a niobiumoxide film inhibits an electrochemical decomposition reaction betweenthe negative electrode active material and an electrolyte solution. Onthe other hand, niobium oxide has a lithium diffusion coefficient of10⁻⁹ cm²/sec and high lithium ion conductivity. Therefore, niobium oxidecan transmit lithium ions.

Thus, having carrier ion conductivity, the inorganic compound film 102provided on part of the surface of the negative electrode activematerial 101 can transmit carrier ions, and a battery reaction of thenegative electrode active material 101 can occur. On the other hand,having an insulating property, the inorganic compound film 102 cansuppress a reaction between an electrolyte solution and the negativeelectrode active material 101. Therefore, it is preferable that amaterial having a high carrier ion diffusion coefficient be used for theinorganic compound film 102 and the inorganic compound film 102 beformed as thin as possible (i.e., be a film having low electronconductivity). The inorganic compound film 102 is preferably thin enoughto transmit carrier ions. Typically, the inorganic compound film 102preferably has a thickness more than or equal to 5 nm and less than orequal to 50 nm.

When the negative electrode active material 101 is entirely isolatedelectrically, electrons are prevented from freely moving inside andoutside the negative electrode active material 101; thus, the batteryreaction cannot occur. Therefore, it is preferable that the negativeelectrode active material 101 be prevented from being completely coveredwith the inorganic compound film 102 and at least part of the negativeelectrode active material 101 be exposed without being covered with theinorganic compound film 102 to ensure a path for electron conductionwith the outside.

The film 103 formed on the exposed portion of the negative electrodeactive material 101 and the surface of the inorganic compound film 102is a reductive decomposition product of a reductive decompositionreaction of an electrolyte solution or a product of a reaction betweenthe reductive decomposition product and the electrolyte solution.Therefore, the film 103 contains an inorganic compound and an organiccompound that is the reductive decomposition product of the reductivedecomposition reaction of the electrolyte solution or the product of thereaction between the reductive decomposition product and the electrolytesolution.

The inorganic compound contained in the film 103 is a fluoride, acarbonate, an oxide, or a hydroxide containing one of lithium, sodium,potassium, calcium, strontium, barium, beryllium, and magnesium.

When the carrier ions are lithium ions, lithium fluoride, lithiumcarbonate, lithium oxide, lithium hydroxide, or the like is given as theinorganic compound contained in the film 103. When the carrier ions aresodium ions, sodium fluoride, sodium carbonate, sodium oxide, sodiumhydroxide, or the like is given as the inorganic compound.

The film 103 provided on the exposed portion of the negative electrodeactive material 101 has high carrier ion conductivity and low electronconductivity. For this reason, when the inorganic compound film 102 isnot provided on the negative electrode active material 101 and the film103 is formed on the surface of the negative electrode active material101, the reductive decomposition reaction of the electrolyte solutioncan be suppressed; however, the suppression is temporary, and the volumeof the negative electrode active material 101 changes and the thicknessof the film 103 increases due to repeated charge and discharge and thuspart of the film 103 is separated.

Therefore, the film 103 is provided on the exposed portion of thenegative electrode active material 101 and the inorganic compound film102 to enable an increase in adhesion between the inorganic compoundfilm 102 and the film 103 in the negative electrode active materiallayer 13. As a result, separation of the film 103 can be suppressed evenwhen charge and discharge are repeated.

The inorganic compound film 102, e.g., niobium oxide (Nb₂O₅), providedon part of the surface of the particulate negative electrode activematerial 101 has a low electron conductivity of 10⁻⁹ S/cm² and a highinsulating property. Thus, a niobium oxide film inhibits theelectrochemical decomposition reaction between the negative electrodeactive material and the electrolyte solution. On the other hand, niobiumoxide has a lithium diffusion coefficient of 10⁻⁹ cm²/sec and highlithium ion conductivity, and therefore can transmit lithium ions.However, the inorganic compound film 102 with an increased thicknessincreases an insulating property, and lithium ions are prevented fromfreely moving inside and outside the negative electrode active material101, which inhibits the battery reaction.

Thus, the inorganic compound film 102 is thinned so that carrier ionscan pass through the inorganic compound film 102 and the film 103 isprovided in contact with the exposed portion of the negative electrodeactive material 101 and the inorganic compound film 102, which makes itpossible to suppress the electron conductivity of the inorganic compoundfilm 102 while the carrier ion conductivity of the inorganic compoundfilm 102 is secured. In other words, it is possible to minimize thereductive decomposition reaction of the electrolyte solution on thesurface of the negative electrode active material; consequently, adecrease in the capacity of the power storage device can be suppressed.

When carrier ions solvated with a solvent of the electrolyte solutionpass through the inorganic compound film 102, they are desolvated, andonly the carrier ions are diffused into the negative electrode activematerial 101. Therefore, an area of a portion where the electrolytesolution is directly in contact with the negative electrode active 101is reduced, which makes it possible to suppress the reductivedecomposition reaction of the electrolyte solution and to suppress anincrease in the thickness of the film 103. Consequently, it is possibleto suppress separation of the film 103 due to repeated charge anddischarge.

As described above, the inorganic compound film 102 is formed on part ofthe surface of the particulate negative electrode active material 101and the film 103 is provided on the exposed portion of the negativeelectrode active material 101 and the inorganic compound film 102; thus,the surface of the negative electrode active material 101 can be stable,and the decomposition reaction of the electrolyte solution can beminimized while the battery reaction of the negative electrode active101 is possible.

This embodiment can be implemented combining with any of otherembodiments as appropriate.

(Embodiment 2)

In this embodiment, a method for forming a particulate negativeelectrode active material having an inorganic compound film on part ofits surface is described with reference to FIG. 2.

First, as Step S150, a solvent to which metal alkoxide and a stabilizingagent are added is stirred to form a solution. Toluene can be used asthe solvent, for example. Ethyl acetoacetate can be used as thestabilizing agent, for example. Alternatively, as Step S150, a solventto which silicon alkoxide and a stabilizing agent are added is stirredto form a solution.

For the metal alkoxide, a desired metal used to form the inorganiccompound film provided on part of the surface of the negative electrodeactive material is selected.

Next, as Step S151, the solution to which a particulate negativeelectrode active material such as graphite is added is stirred. Thesolution is made into thick paste by stirring the solution to which asolvent such as toluene is added, and the metal alkoxide or the siliconalkoxide is provided on part of the surface of the negative electrodeactive material. Step S150 and Step S151 are preferably performed in anenvironment at low humidity, such as a dry room. This is because ahydrolysis reaction can be suppressed.

Next, in Step S152 and Step S153, the metal alkoxide or the siliconalkoxide on the surface of the particulate negative electrode activematerial is changed into a gel by a sol-gel method.

As Step S152, a small amount of water is added to the solution to whichthe negative electrode active material such as graphite is added, sothat the metal alkoxide or the silicon alkoxide reacts with the water(i.e., hydrolysis reaction) to form a decomposition product which is asol. Here, the term “being a sol” refers to being in a state where solidfine particles are substantially uniformly dispersed in a liquid. Thesmall amount of water may be added by exposing the solution to which thenegative electrode active material is added to the air. For example, inthe case where Nb(OEt)₅ which is one of niobium alkoxide is used as themetal alkoxide, a hydrolysis reaction represented by Formula 1 occurs.Nb(OEt)₅+5H₂O→Nb(OEt)_(5−x)(OH)_(x) +xEtOH+(5−x)H₂O (x is a positivenumber of 5 or less)  [Formula 1]

Alternatively, for example, in the case where Si(OEt)₄ which is one ofsilicon alkoxide is used, a hydrolysis reaction represented by Formula 2occurs.Si(OEt)₄+4H₂O→Si(OEt)_(4−x)(OH)_(x) +xEtOH+(4−x)H₂O (x is a positivenumber of 4 or less)  [Formula 2]

Next, as Step S153, the decomposition product changed into the sol isdehydrated and condensed to be a reactant which is a gel. Here, “being agel” refers to being in a state where a decomposition product which is asol and has fluidity is solidified, and a three-dimensional networkstructure is developed due to attractive interaction between solid fineparticles. In the case where Nb(OEt)₅ which is one of niobium alkoxideis used as the metal alkoxide, a condensation reaction represented byFormula 3 occurs.2Nb(OEt)_(5−x)(OH)_(x)→[Nb(OEt)_(5−x)(OH)_(x−1)]—O—[Nb(OEt)_(5−x)(OH)_(x−1)]+H₂O(x is a positive number of 5 or less)  [Formula 3]

Alternatively, for example, in the case where Si(OEt)₄ which is one ofsilicon alkoxide is used, a condensation reaction represented by Formula4 occurs.2Si(OEt)_(4−x)(OH)_(x)→(OEt)_(4−x)(OH)_(x−1)Si—O—Si(OH)_(x−1)(OEt)_(4−x)+H₂O(x is a positive number of 4 or less)  [Formula 4]

Through this step, the reactant which is a gel attached on the surfaceof the particulate negative electrode active material can be formed.Note that although the solation by the hydrolysis reaction and thegelation by the condensation reaction are separately described above astwo steps, Steps S152 and S153, for convenience, both reactions occuralmost at the same time in practice. This is because the structure ofmetal alkoxide or silicon alkoxide gradually changes into that of astable substance which is a gel, depending on conditions of temperatureand water.

Then, as Step S154, the dispersion liquid is baked under an atmosphericpressure, whereby the particulate negative electrode active materialwith a metal oxide film or a silicon oxide film attached on the surfacethereof can be obtained. The temperature of the baking is more than orequal to 300° C. and less than or equal to 900° C., preferably more thanor equal to 500° C. and less than or equal to 800° C.

Through the above steps, a negative electrode active material having aninorganic compound film, which contains a metal oxide film or a siliconoxide film as a component, on part of its surface is formed. In the caseof forming an inorganic compound film on a negative electrode activematerial by a sol-gel method in such a manner, the above steps can beeven employed for a negative electrode active material having acomplicated shape, and a large number of inorganic compound films can beformed; therefore, the method for forming a negative electrode activematerial described in this embodiment is an optimal method for a massproduction process.

(Embodiment 3)

In this embodiment, a power storage device including the negativeelectrode described in Embodiment 1 and a method for forming the powerstorage device including the negative electrode are described withreference to FIGS. 3A to 3D, FIGS. 4A to 4C, FIGS. 5A and 5B, FIG. 6,and FIGS. 7A and 7B.

First, a negative electrode for a power storage device using aparticulate negative electrode active material having an inorganiccompound film on part of its surface and a method for forming thenegative electrode are described with reference to FIGS. 3A to 3D.

As illustrated in FIG. 3A, a negative electrode 200 includes a negativeelectrode current collector 201 and a negative electrode active materiallayer 202 provided on one or both surfaces (on the both surfaces in thedrawing) of the negative electrode current collector 201.

The negative electrode current collector 201 is formed using a highlyconductive material which is not alloyed with a carrier ion such aslithium. For example, stainless steel, iron, aluminum, copper, nickel,or titanium can be used. Alternatively, an alloy material such as analloy of aluminum and nickel or an alloy of aluminum and copper may beused. In addition, the negative electrode current collector 201 can havea foil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a punching-metal shape, an expanded-metal shape, or the like asappropriate. The negative electrode current collector 201 preferably hasa thickness of more than or equal to 10 μm and less than or equal to 30μm.

The negative electrode active material layer 202 is provided on one orboth surfaces of the negative electrode current collector 201. Thenegative electrode active material layer 202 includes the particulatenegative electrode active material having the inorganic compound film onpart of its surface and the film in contact with the exposed portion ofthe negative electrode active material and the inorganic compound film,which are described in Embodiment 1 or 2.

The negative electrode active material layer 202 is described withreference to FIG. 3B. FIG. 3B is a cross-sectional view of part of thenegative electrode active material layer 202. The negative electrodeactive material layer 202 includes a particulate negative electrodeactive material 203 described in Embodiment 1 or 2, a conductiveadditive 204, and a binder (not illustrated). The particulate negativeelectrode active material 203 has an inorganic compound film on part ofits surface as described in the above embodiments. In addition, in partof the negative electrode active material layer 202, a film (notillustrated) is provided on an exposed portion of the negative electrodeactive material 203 and the inorganic compound film.

The conductive additive 204 increases the conductivity between particlesof the negative electrode active material 203 or between the negativeelectrode active material 203 and the negative electrode currentcollector 201, and is preferably added to the negative electrode activematerial layer 202. A material with a large specific surface isdesirably used as the conductive additive 204, and acetylene black (AB)or the like is preferably used. Alternatively, a carbon material such asa carbon nanotube, graphene, or fullerene can be used. Note that thecase of using graphene is described later as an example.

As the binder, a material which at least binds the negative electrodeactive material, the conductive additive, and the current collector isused. Examples of the binder include resin materials such aspolyvinylidene fluoride (PVdF), a vinylidenefluoride-hexafluoropropylene copolymer, a vinylidenefluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymerrubber, polytetrafluoroethylene, polypropylene, polyethylene, andpolyimide.

The negative electrode 200 is formed in the following manner. First, theparticulate negative electrode active material having the inorganiccompound film formed by the method described in Embodiment 2 is mixedwith a solvent such as NMP (N-methylpyrrolidone), in which a vinylidenefluoride based polymer such as polyvinylidene fluoride is dissolved, toform slurry.

Next, the slurry is applied on one or both of the surfaces of thenegative electrode current collector 201, and dried. In the case whereboth surfaces of the negative electrode current collector 201 aresubjected to the application step, the slurry is applied to the bothsurfaces at the same time or one by one, and dried. Then, rolling with aroller press machine is performed, and thus a structure body for anegative electrode in which the particulate negative electrode activematerial having the inorganic compound film on part of its surface isprovided on one or both of the surfaces of the negative electrodecurrent collector 201 is formed.

Next, the structure body for a negative electrode and a referenceelectrode of lithium or the like are immersed in an electrolyte solutiondescribed later, and voltage is applied to the negative electrodecurrent collector 201 and the reference electrode. As a result, in thestructure body for a negative electrode, a film is formed on an exposedportion of the negative electrode active material and the inorganiccompound film, which are in contact with the electrolyte solution. Inother words, the negative electrode 200 in which the negative electrodeactive material layer, which includes the particulate negative electrodeactive material having the inorganic compound film on part of itssurface and the film in contact with the exposed portion of the negativeelectrode active material and the inorganic compound film, is providedover the negative electrode current collector 201 can be formed.

Next, an example of using graphene as the conductive additive added tothe negative electrode active material layer 202 is described withreference to FIGS. 3C and 3D.

Here, graphene in this specification includes single-layer graphene andmultilayer graphene including two to hundred layers. Single-layergraphene refers to a sheet of a monolayer of carbon molecules. Grapheneoxide refers to a compound formed by oxidation of such graphene. Whengraphene oxide is reduced to form graphene, oxygen contained in thegraphene oxide is not entirely extracted and part of the oxygen remainsin the graphene. When the graphene contains oxygen, the proportion ofoxygen is higher than or equal to 2 atomic % and lower than or equal to20 atomic %, preferably higher than or equal to 3 atomic % and lowerthan or equal to 15 atomic %.

Here, in the case of reducing multilayer graphene oxide to obtainmultilayer graphene, an interlayer distance of the multilayer grapheneis greater than or equal to 0.34 nm and less than or equal to 0.5 nm,preferably greater than or equal to 0.38 nm and less than or equal to0.42 nm, more preferably greater than or equal to 0.39 nm and less thanor equal to 0.41 nm. In other words, the multilayer graphene used in apower storage device of one embodiment of the present invention can havean interlayer distance longer than 0.34 nm that is the interlayerdistance of general graphite. Since the multilayer graphene used in thepower storage device of one embodiment of the present invention can havea long interlayer distance, carrier ions can easily transfer betweenlayers of the multilayer graphene.

FIG. 3C is a plan view of part of the negative electrode active materiallayer 202 using graphene. The negative electrode active material layer202 includes the particulate negative electrode active material 203having the inorganic compound film on part of its surface and graphenes205 which cover a plurality of particles of the negative electrodeactive material 203 and at least partly surround the plurality ofparticles of negative electrode active material 203. In addition, thenegative electrode active material layer 202 includes the particulatenegative electrode active material having the inorganic compound film onpart of its surface and the film (not illustrated) which is in contactwith an exposed portion of the negative electrode active material, theinorganic compound film, and the graphene. The binder which is notillustrated may be added. However, the binder is not necessarily addedin the case where the graphenes 205 are contained so that they are boundto each other to be fully functional as a binder. The differentgraphenes 205 cover surfaces of the plurality of particles of thenegative electrode active material 203 in the negative electrode activematerial layer 202 in the plan view. The particles of the negativeelectrode active material 203 may partly be exposed.

FIG. 3D is a cross-sectional view of part of the negative electrodeactive material layer 202 in FIG. 3C. In FIG. 3D, the graphenes 205cover a plurality of particles of the negative electrode active material203 in the negative electrode active material layer 202 in the planview. The graphenes 205 are observed to have linear shapes in thecross-sectional view. One graphene or plural graphenes overlap with theplurality of particles of the negative electrode active material 203, orthe plurality of particles of the negative electrode active material 203exists within one graphene or plural graphenes. Note that the graphene205 has a bag-like shape and the plurality of particles of the negativeelectrode active material is at least partly surrounded with thebag-like portion in some cases. The graphene 205 partly has openingswhere the particles of the negative electrode active material 203 areexposed in some cases.

The desired thickness of the negative electrode active material layer202 is determined in the range of 20 μm to 150 μm.

Note that the negative electrode active material layer 202 may bepredoped with lithium. Predoping with lithium may be performed in such amanner that a lithium layer is formed on a surface of the negativeelectrode active material layer 202 by a sputtering method.Alternatively, lithium foil is provided on the surface of the negativeelectrode active material layer 202, whereby the negative electrodeactive material layer 202 can be predoped with lithium.

As an example of the negative electrode active material 203, there is amaterial whose volume is increased by occlusion of carrier ions. Thus,the negative electrode active material layer containing such a materialgets friable and is partly broken due to charge and discharge, whichreduces the reliability (e.g., cycle performance) of the power storagedevice. However, even when the volume of the negative electrode activematerial increases due to charge and discharge, the graphene partlycovers the periphery of the negative electrode active material, whichallows prevention of dispersion of the particles of the negativeelectrode active material and the breakdown of the negative electrodeactive material layer. That is to say, the graphene has a function ofmaintaining the bond between the particles of the negative electrodeactive material even when the volume of the negative electrode activematerial fluctuates due to charge and discharge.

The graphene 205 has conductivity and is in contact with the pluralityof particles of the negative electrode active material 203; thus, italso serves as a conductive additive. That is, a conductive additivedoes not have to be mixed into forming the negative electrode activematerial layer 202. Accordingly, the proportion of the negativeelectrode active material in the negative electrode active materiallayer 202 with certain weight (certain volume) can be increased, leadingto an increase in capacity per unit weight (unit volume) of theelectrode.

Further, the graphene 205 efficiently forms a sufficient conductive pathof electrons in the negative electrode active material layer 202, whichincreases the conductivity of the negative electrode for a power storagedevice.

Note that the graphene 205 also functions as a negative electrode activematerial that can occlude and release carrier ions, leading to anincrease in capacity of the negative electrode for a power storagedevice which is formed later.

Next, a method for forming the negative electrode active material layer202 in FIGS. 3C and 3D is described.

First, the particulate negative electrode active material 203 having theinorganic compound film on part of its surface, which is described inEmbodiment 1 or 2, and a dispersion liquid containing graphene oxide aremixed to form slurry.

Next, the slurry is applied to one or both of surfaces of the negativeelectrode current collector 201, and is dried. Then, rolling with aroller press machine is performed.

Then, the graphene oxide is electrochemically reduced with electricenergy or thermally reduced by heat treatment to form the graphene 205.In addition, a structure body for a negative electrode including thenegative electrode current collector 201 and the particulate negativeelectrode active material having the inorganic compound film on part ofits surface is formed. Particularly in the case of performingelectrochemical reduction treatment, a proportion of C(π)—C(π) doublebonds of graphene formed by the electrochemical reduction treatment ishigher than that of graphene formed by heat treatment; therefore, thegraphene 205 having high conductivity can be formed.

Next, the structure body for a negative electrode and the referenceelectrode of lithium or the like are immersed in the electrolytesolution described later, and voltage is applied to the negativeelectrode current collector 201 and the reference electrode. As aresult, in the structure body for a negative electrode, a film is formedon an exposed portion of the negative electrode active material and theinorganic compound film, which are in contact with the electrolytesolution. In other words, the negative electrode 200 in which thenegative electrode active material layer, which includes the particulatenegative electrode active material having the inorganic compound film onpart of its surface, the graphene, and the film in contact with theexposed portion of the negative electrode active material, the inorganiccompound film, and the graphene, is provided over the negative electrodecurrent collector 201 can be formed.

Through the above steps, the negative electrode active material layer202 in which the graphene is used as a conductive additive can be formedon one or both of the surfaces of the negative electrode currentcollector 201, and thus the negative electrode 200 can be formed.

Next, a positive electrode and a method for forming the positiveelectrode method are described with reference to FIGS. 4A to 4C.

FIG. 4A is a cross-sectional view of a positive electrode 250. In thepositive electrode 250, a positive electrode active material layer 252is formed over a positive electrode current collector 251.

For the positive electrode current collector 251, a highly conductivematerial such as a metal typified by stainless steel, gold, platinum,zinc, iron, copper, aluminum, or titanium, or an alloy of these metalscan be used. Note that the positive electrode current collector 251 canbe formed using an aluminum-alloy to which an element which improvesheat resistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. Further alternatively, the positive electrodecurrent collector 251 may be formed using a metal element which formssilicide by reacting with silicon. Examples of the metal element whichforms silicide by reacting with silicon include zirconium, titanium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,cobalt, nickel, and the like. The positive electrode current collector251 can have a foil-like shape, a plate-like shape (a sheet-like shape),a net-like shape, a punching-metal shape, an expanded-metal shape, orthe like as appropriate.

In addition to a positive electrode active material, a conductiveadditive and a binder may be included in the positive electrode activematerial layer 252.

As the positive electrode active material of the positive electrodeactive material layer 252, a material that can insert and extractcarrier ions such as lithium ions can be used. For example, alithium-containing composite oxide with an olivine crystal structure, alayered rock-salt crystal structure, or a spinel crystal structure canbe given as the positive electrode active material that can insert andextract lithium ions.

As the lithium-containing composite oxide with an olivine crystalstructure, a composite oxide (represented by a general formula LiMPO₄ (Mis one or more of Fe(II), Mn(II), Co(II), and Ni(II))) can be given.Typical examples of the general formula LiMPO₄ include LiFePO₄, LiNiPO₄,LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄,LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1,and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄(c+d+e≦1, 0<c<1, 0<d<1, and 0<e<1),LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1, 0<h<1, and 0<i<1), and the like.

LiFePO₄ is particularly preferable because it meets requirements withbalance for a positive electrode active material, such as safety,stability, high capacity density, high potential, and the existence oflithium ions that can be extracted in initial oxidation (charging).

Examples of the lithium-containing composite oxide with a layeredrock-salt crystal structure include lithium cobalt oxide (LiCoO₂);LiNiO₂; LiMnO₂; Li₂MnO₃; an NiCo-based lithium-containing compositeoxide (a general formula thereof is LiNi_(x)Co_(1−x)O₂ (0<x<1)) such asLiNi_(o.8)Co_(0.2)O₂; an NiMn-based lithium-containing composite oxide(a general formula thereof is LiNi_(x)Mn_(1−x)O₂ (0<x<1)) such asLiNi_(0.5)Mn_(0.5)O₂; and an NiMnCo-based lithium-containing compositeoxide (also referred to as NMC, and a general formula thereof isLiNi_(x)Mn_(y)Co_(1−x−y)O₂ (x>0, y>0, x+y<1)) such asLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂. Moreover,Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂, Li₂MnO₃—LiMO₂ (M=Co, Ni, or Mn), andthe like can be given.

LiCoO₂ is particularly preferable because it has high capacity, is morestable in the air than LiNiO₂, and is more thermally stable than LiNiO₂,for example.

Examples of the lithium-containing composite oxide with a spinel crystalstructure include LiMn₂O₄, Li_(1+x)Mn_(2−x)O₄, Li(MnAl)₂O₄,LiMn_(1.5)Ni_(0.5)O₄, and the like.

A lithium-containing composite oxide with a spinel crystal structureincluding manganese, such as LiMn₂O₄, is preferably mixed with a smallamount of lithium nickel oxide (e.g., LiNiO₂ or LiNi_(1−x)MO₂ (M=Co, Al,or the like)), in which case elution of manganese and decomposition ofan electrolyte solution are suppressed, for example.

Alternatively, as the positive electrode active material, a compositeoxide represented by a general formula Li(_(2−j))MSiO₄ (M is one or moreof Fe(II), Mn(II), Co(II), and Ni(II); 0≦j 2) can be used. Typicalexamples of the general formula Li(_(2−j))MSiO₄ includeLi(_(2−j))FeSiO₄, Li(_(2−j))NiSiO₄, Li(_(2−j))CoSiO₄, Li(_(2−j))MnSiO₄,Li(_(2−j))Fe_(k)Ni_(l)SiO₄, Li(_(2−j))Fe_(k)Co_(l)SiO₄,Li(_(2−j))Fe_(k)Mn_(l)SiO₄, Li(_(2−j))Ni_(k)Co_(l)SiO₄,Li(_(2−j))Ni_(k)Mn_(l)SiO₄ (k+l≦1, 0<k<1, and 0<1<1),Li(_(2−j))Fe_(m)Ni_(n)Co_(q)SiO₄, Li(_(2−j))Fe_(m)Ni_(n)MN_(q)SiO₄,Li(_(2−j))Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≦1, 0<m<1, 0<n<1, and 0<q<1),Li(_(2−J))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≦1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1), and the like.

Further alternatively, as the positive electrode active material, anasicon compound represented by a general formula A_(x)M₂(XO₄)₃ (A=Li,Na, or Mg; M=Fe, Mn, Ti, V, Nb, or Al; and X=S, P, Mo, W, As, or Si) canbe used. Examples of the nasicon compound include Fe₂(MnO₄)₃, Fe₂(SO₄)₃,Li₃Fe₂(PO₄)₃, and the like. Still further alternatively, as the positiveelectrode active material, a compound represented by a general formulaLi₂MPO₄F, Li₂MP₂O₇, or Li₅MO₄ (M=Fe or Mn); perovskite fluoride such asNaF₃ or FeF₃; metal chalcogenide such as TiS₂ or MoS₂ (sulfide,selenide, or telluride); a lithium-containing composite oxide with aninverse spinel crystal structure such as LiMVO₄; a vanadium oxide basedmaterial (e.g., V₂O₅, V₆O₁₃, or LiV₃O₈); a manganese oxide basedmaterial; an organic sulfur based material; or the like can be used.

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions, thepositive electrode active material layer 252 may contain, instead oflithium in the lithium compound and the lithium-containing compositeoxide, an alkali metal (e.g., sodium or potassium), an alkaline-earthmetal (e.g., calcium, strontium, or barium), beryllium, or magnesium.

The positive electrode active material layer 252 is not necessarilyformed in contact with the positive electrode current collector 251.Between the positive electrode current collector 251 and the positiveelectrode active material layer 252, any of the following functionallayers may be formed using a conductive material such as a metal: anadhesive layer for the purpose of improving adhesiveness between thepositive electrode current collector 251 and the positive electrodeactive material layer 252, a planarization layer for reducing unevennessof the surface of the positive electrode current collector 251, a heatradiation layer for radiating heat, and a stress relaxation layer forrelieving stress of the positive electrode current collector 251 or thepositive electrode active material layer 252.

FIG. 4B is a plan view of the positive electrode active material layer252. As the positive electrode active material layer 252, a particulatepositive electrode active material 253 that can occlude and releasecarrier ions is used. An example is shown in which graphenes 254covering a plurality of particles of the positive electrode activematerial 253 and at least partly surrounding the plurality of particlesof the positive electrode active material 253 are included. Thedifferent graphenes 254 cover surfaces of the plurality of particles ofthe positive electrode active material 253. The particles of thepositive electrode active material 253 may partly be exposed.

The size of the particle of the positive electrode active material 253is preferably greater than or equal to 20 nm and less than or equal to100 nm. Note that the size of the particle of the positive electrodeactive material 253 is preferably smaller because electrons transfer inthe positive electrode active material 253.

Although sufficient characteristics can be obtained even when thesurface of the positive electrode active material 253 is not coveredwith a graphite layer, graphene and a positive electrode active materialcovered with a graphite layer are preferably used, in which case hoppingof carrier ions occurs between particles of the positive electrodeactive material, so that current flows.

FIG. 4C is a cross-sectional view of part of the positive electrodeactive material layer 252 in FIG. 4B. The positive electrode activematerial layer 252 includes the positive electrode active material 253and the graphenes 254 covering a plurality of particles of the positiveelectrode active material 253. The graphene 254 has a linear shape whenobserved in the cross-sectional view. The plurality of particles of thepositive electrode active material is at least partly surrounded withone graphene or plural graphenes, or the plurality of particles of thepositive electrode active material exists within one graphene or pluralgraphenes. Note that the graphene has a bag-like shape and the pluralityof particles of the positive electrode active material is at leastpartly surrounded with the bag-like portion in some cases. In addition,the particles of the positive electrode active material are partly notcovered with the graphenes and exposed in some cases.

The desired thickness of the positive electrode active material layer252 is determined in the range of 20 μm to 100 μm. It is preferable toadjust the thickness of the positive electrode active material layer 252as appropriate so that cracks and separation do not occur.

Note that the positive electrode active material layer 252 may contain aknown conductive additive, for example, acetylene black particles havinga volume 0.1 to 10 times as large as that of the graphene or carbonparticles such as carbon nanofibers having a one-dimensional expansion.

As an example of a material of the positive electrode active material, amaterial whose volume is increased by occlusion of ions serving ascarriers is given. When such a material is used, the positive electrodeactive material layer gets friable and is partly broken due to chargeand discharge, which results in lower reliability of the power storagedevice. However, even when the volume of the positive electrode activematerial is increased due to charge and discharge, the graphene partlycovers the periphery of the positive electrode active material, whichallows prevention of dispersion of the particles of the positiveelectrode active material and the breakdown of the positive electrodeactive material layer. That is to say, the graphene has a function ofmaintaining the bond between the particles of the positive electrodeactive material even when the volume of the positive electrode activematerial fluctuates due to charge and discharge.

The graphene 254 is in contact with the plurality of particles of thepositive electrode active material 253 and serves also as a conductiveadditive. Further, the graphene 254 has a function of holding thepositive electrode active material capable of occluding and releasingcarrier ions. Thus, a binder does not have to be mixed into the positiveelectrode active material layer. Accordingly, the amount of the positiveelectrode active material in the positive electrode active materiallayer can be increased, which allows an increase in capacity of thepower storage device.

Next, description is given of a method for forming the positiveelectrode active material layer 252.

First, slurry containing the particulate positive electrode activematerial and graphene oxide is formed. Next, the slurry is applied ontothe positive electrode current collector 251. Then, heating is performedin a reduced atmosphere for reduction treatment so that the positiveelectrode active material is baked and oxygen included in the grapheneoxide is extracted to form graphene. Note that oxygen in the grapheneoxide is not entirely extracted and partly remains in the graphene.Through the above steps, the positive electrode active material layer252 can be formed over the positive electrode current collector 251.Consequently, the positive electrode active material layer 252 hashigher conductivity.

Graphene oxide contains oxygen and thus is negatively charged in a polarsolvent. As a result of being negatively charged, graphene oxide isdispersed in the polar solvent. Therefore, the particles of the positiveelectrode active material contained in the slurry are not easilyaggregated, so that an increase in the size of the particle of thepositive electrode active material due to aggregation can be prevented.Thus, the transfer of electrons in the positive electrode activematerial is facilitated, resulting in an increase in conductivity of thepositive electrode active material layer.

Next, the power storage device and a method for manufacturing the powerstorage device are described. Here, a structure and a method formanufacturing a lithium ion battery, which is one mode of the powerstorage device, are described with reference to FIGS. 5A and 5B. Here, across-sectional structure of the lithium ion battery is described below.

(Coin-type Lithium Ion Battery)

FIG. 5A is an external view of a coin-type (single-layer flat type)lithium ion battery, and FIG. 5B is a cross-sectional view thereof.

In a coin-type lithium ion battery 300, a positive electrode can 301serving also as a positive electrode terminal and a negative electrodecan 302 serving also as a negative electrode terminal are insulated andsealed with a gasket 303 formed of polypropylene or the like. A positiveelectrode 304 includes a positive electrode current collector 305 and apositive electrode active material layer 306 which is provided to be incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 which is provided to be incontact with the negative electrode current collector 308. A separator310 and an electrolyte solution (not illustrated) are included betweenthe positive electrode active material layer 306 and the negativeelectrode active material layer 309.

As the negative electrode 307, the negative electrode 10 described inEmbodiment 1 is used. As the positive electrode 304, the positiveelectrode 250 described in this embodiment can be used.

For the separator 310, an insulator such as cellulose (paper),polypropylene with pores, or polyethylene with pores can be used.

As an electrolyte of the electrolyte solution, a material which containscarrier ions is used. Typical examples of the electrolyte includelithium salts such as LiClO₄, LiAsF₆, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), LiN(C₂F₅SO₂)₂, andthe like.

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions, theelectrolyte may contain, instead of lithium in the lithium salts, analkali metal (e.g., sodium or potassium), an alkaline-earth metal (e.g.,calcium, strontium, or barium), beryllium, or magnesium.

As a solvent for the electrolyte solution, a material that can transfercarrier ions is used. As the solvent for the electrolyte solution, anaprotic organic solvent is preferably used. Typical examples of theaprotic organic solvent include ethylene carbonate (EC), propylenecarbonate, butylene carbonate, chloroethylene carbonate, vinylenecarbonate, dimethyl carbonate, diethyl carbonate (DEC), ethyl methylcarbonate (EMC), γ-butyrolactone, γ-valerolactone, methyl formate,methyl acetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethylsulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile,dimethoxyethane, tetrahydrofuran, sulfolane, and sultone, and one ormore of these materials can be used. When a gelled high-molecularmaterial is used as the solvent for the electrolyte solution, safetyagainst liquid leakage and the like is improved. Further, a lithium ionbattery can be thinner and more lightweight. Typical examples of thegelled high-molecular material include a silicone gel, an acrylic gel,an acrylonitrile gel, polyethylene oxide, polypropylene oxide, afluorine-based polymer, and the like. Alternatively, the use of one ormore of ionic liquids (room temperature molten salts) which are lesslikely to burn and volatilize as the solvent for the electrolytesolution can prevent a lithium ion battery from exploding or catchingfire even when the lithium ion battery internally shorts out or theinternal temperature increases due to overcharging or the like.

Instead of the electrolyte solution, a solid electrolyte including asulfide-based inorganic material, an oxide-based inorganic material, orthe like, or a solid electrolyte including a polyethylene oxide(PEO)-based high-molecular material or the like can be used. In the caseof using the solid electrolyte, a separator is not necessary. Further,the battery can be entirely solidified; therefore, there is nopossibility of liquid leakage and thus the safety of the battery isdramatically increased.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to a liquid (e.g., anelectrolyte solution) in charging and discharging a secondary battery,such as nickel, aluminum, or titanium; an alloy of any of the metals; analloy containing any of the metals and another metal (e.g., stainlesssteel); a stack of any of the metals; a stack including any of themetals and any of the alloys (e.g., a stack of stainless steel andaluminum); or a stack including any of the metals and another metal(e.g., a stack of nickel, iron, and nickel) can be used. The positiveelectrode can 301 and the negative electrode can 302 are electricallyconnected to the positive electrode 304 and the negative electrode 307,respectively.

Next, the method for manufacturing the power storage device isdescribed.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte solution. Then, asillustrated in FIG. 5B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303provided therebetween. In such a manner, the coin-type lithium ionbattery 300 is manufactured.

Note that instead of the negative electrode 307, the structure body fora negative electrode described in the method for forming the negativeelectrode 200 illustrated in FIGS. 3A to 3D is used, and the structurebody, the positive electrode 304, and a separator are immersed in anelectrolyte solution. A positive electrode can, the positive electrode,the separator, the structure body for a negative electrode, and anegative electrode can are stacked in this order with the positiveelectrode can positioned at the bottom. The positive electrode can andthe negative electrode can are subjected to pressure bonding with agasket provided therebetween. Next, voltage within a predetermined rangeis applied to the positive electrode can and the negative electrode canto perform charge and discharge, so that due to a reductivedecomposition reaction of the electrolyte solution, in the structurebody for a negative electrode, a film is formed on an exposed portion ofthe negative electrode active material and the inorganic compound film,which are in contact with the electrolyte solution. In other words, thenegative electrode 307 in which the negative electrode active materiallayer, which includes the particulate negative electrode active materialhaving the inorganic compound film on part of its surface and the filmin contact with the exposed portion of the negative electrode activematerial and the inorganic compound film, is provided over the negativeelectrode current collector can be formed and the coin-type lithium ionbattery 300 can be manufactured. In the method for manufacturing thelithium ion battery, the negative electrode can be formed while thelithium ion battery is manufactured, which makes it possible to reducethe number of manufacturing steps of the lithium ion battery.

(Laminated Lithium Ion Battery)

Next, an example of a laminated lithium ion battery is described withreference to FIG. 6.

In a laminated lithium ion battery 400 illustrated in FIG. 6, a positiveelectrode 403 including a positive electrode current collector 401 and apositive electrode active material layer 402, a separator 407, and anegative electrode 406 including a negative electrode current collector404 and a negative electrode active material layer 405 are stacked andsealed in an exterior body 409, and then an electrolyte solution 408 isinjected into the exterior body 409. Although the laminated lithium ionbattery 400 in FIG. 6 has a structure where one sheet-like positiveelectrode 403 and one sheet-like negative electrode 406 are stacked, itis preferable to roll the stack or to stack a plurality of pieces of thestacks and then laminate them in order to increase battery capacity.Particularly in the case of the laminated lithium ion battery, thebattery has flexibility and thus is suitable for applications whichrequire flexibility.

In the laminated lithium ion battery 400 illustrated in FIG. 6, thepositive electrode current collector 401 and the negative electrodecurrent collector 404 serve as terminals for an electrical contact withthe outside. For this reason, the positive electrode current collector401 and the negative electrode current collector 404 are arranged sothat part of the positive electrode current collector 401 and part ofthe negative electrode current collector 404 are exposed outside theexterior body 409.

As the exterior body 409 in the laminated lithium ion battery 400, forexample, a laminate film having a three-layer structure in which ahighly flexible metal thin film of aluminum, stainless steel, copper,nickel, or the like is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, or polyamide, andan insulating synthetic resin film of a polyamide-based resin, apolyester-based resin, or the like is provided as the outer surface ofthe exterior body over the metal thin film can be used. With such athree-layer structure, permeation of an electrolytic solution and a gascan be blocked and an insulating property and resistance to theelectrolytic solution can be provided.

Note that in the laminated lithium ion battery 400, the positiveelectrode 403 and the negative electrode 406 may be formed in a mannersimilar to those of the positive electrode and the negative electrode inthe above coin-type lithium ion battery.

(Cylindrical Lithium Ion Battery)

Next, an example of a cylindrical lithium ion battery is described withreference to FIGS. 7A and 7B. As illustrated in FIG. 7A, a cylindricallithium ion battery 500 includes a positive electrode cap (battery lid)501 on its top surface and a battery can (exterior can) 502 on its sidesurface and bottom surface. The positive electrode cap 501 and thebattery can 502 are insulated from each other by a gasket 510(insulating packing).

FIG. 7B is a diagram schematically illustrating a cross section of thecylindrical lithium ion battery. In the battery can 502 with a hollowcylindrical shape, a battery element is provided in which a strip-likepositive electrode 504 and a strip-like negative electrode 506 are woundwith a separator 505 provided therebetween. Although not illustrated,the battery element is wound around a center pin as a center. One end ofthe battery can 502 is close and the other end thereof is open. For thebattery can 502, a metal having a corrosion-resistant property to aliquid (e.g., an electrolyte solution) in charging and discharging asecondary battery, such as nickel, aluminum, or titanium; an alloy ofany of the metals; an alloy containing any of the metals and anothermetal (e.g., stainless steel); a stack of any of the metals; a stackincluding any of the metals and any of the alloys (e.g., a stack ofstainless steel and aluminum); or a stack including any of the metalsand another metal (e.g., a stack of nickel, iron, and nickel) can beused. Inside the battery can 502, the battery element in which thepositive electrode, the negative electrode, and the separator are woundis provided between a pair of insulating plates 508 and 509 which faceeach other. Further, an electrolyte solution (not illustrated) isinjected inside the battery can 502 in which the battery element isprovided. An electrolyte solution which is similar to that of thecoin-type lithium ion battery or the laminated lithium ion battery canbe used.

Although the positive electrode 504 and the negative electrode 506 canbe formed in a manner similar to those of the positive electrode and thenegative electrode of the coin-type lithium ion battery, the differencelies in that, since the positive electrode and the negative electrode ofthe cylindrical lithium ion battery are wound, active materials areformed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 503 is connectedto the positive electrode 504, and a negative electrode terminal(negative electrode current collecting lead) 507 is connected to thenegative electrode 506. A metal material such as aluminum can be usedfor both the positive electrode terminal 503 and the negative electrodeterminal 507. The positive electrode terminal 503 is resistance-weldedto a safety valve mechanism 512, and the negative electrode terminal 507is resistance-welded to the bottom of the battery can 502. The safetyvalve mechanism 512 is electrically connected to the positive electrodecap 501 through a positive temperature coefficient (PTC) element 511.The safety valve mechanism 512 cuts off electrical connection betweenthe positive electrode cap 501 and the positive electrode 504 when theinternal pressure of the battery increases and exceeds a predeterminedthreshold value. The PTC element 511 is a heat sensitive resistor whoseresistance increases as temperature rises, and controls the amount ofcurrent by increase in resistance to prevent unusual heat generation.Barium titanate (BaTiO₃)-based semiconductor ceramic or the like can beused for the PTC element.

Note that in this embodiment, the coin-type lithium ion battery, thelaminated lithium ion battery, and the cylindrical lithium ion batteryare given as examples of the lithium ion battery; however, any oflithium ion batteries with various shapes, such as a sealing-typelithium ion battery and a square-type lithium ion battery, can be used.Further, a structure in which a plurality of positive electrodes, aplurality of negative electrodes, and a plurality of separators arestacked or wound may be employed.

The negative electrode for a power storage device which is oneembodiment of the present invention is used as the negative electrode ineach of the lithium ion battery 300, the lithium ion battery 400, andthe lithium ion battery 500 described in this embodiment. Thus, thelithium ion battery 300, the lithium ion battery 400, and the lithiumion battery 500 can have favorable long-term cycle performance.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

(Embodiment 4)

In this embodiment, a lithium ion capacitor is described as a powerstorage device.

The lithium ion capacitor is a hybrid capacitor which combines apositive electrode of an electrical double layer capacitor (EDLC) and anegative electrode of a lithium ion battery using a carbon material, andis also an asymmetric capacitor in which the principles of power storageare different between the positive electrode and the negative electrode.The positive electrode forms an electrical double layer and enablescharge and discharge by a physical action, whereas the negativeelectrode enables charge and discharge by a chemical action of lithium.With the use of a negative electrode in which lithium is occluded in anegative electrode active material such as a carbon material in advance,the lithium ion capacitor can have energy density dramatically higherthan that of a conventional electrical double layer capacitor includinga negative electrode using active carbon.

In the lithium ion capacitor, instead of the positive electrode activematerial layer in any of the lithium ion battery described in Embodiment3, a material that can occlude at least one of lithium ions and anionsreversibly may be used. Examples of such a material include activecarbon, a conductive high molecule, a polyacene-based organicsemiconductor (PAS), and the like.

The lithium ion capacitor has high efficiency of charge and discharge,capability of rapidly performing charge and discharge, and a long lifeeven when it is repeatedly used.

As the negative electrode of such a lithium ion capacitor, the negativeelectrode for a power storage device which is described in Embodiment 1is used. Thus, a decomposition reaction of an electrolyte solution andthe like caused as a side reaction of charge and discharge can beminimized and therefore, a power storage device having long-term cycleperformance can be manufactured.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

(Embodiment 5)

A power storage device of one embodiment of the present invention can beused as a power supply of various electronic appliances which are drivenby electric power.

Specific examples of electronic appliances each using the power storagedevice of one embodiment of the present invention are as follows:display devices of televisions, monitors, and the like, lightingdevices, desktop personal computers and laptop personal computers, wordprocessors, image reproduction devices which reproduce still images andmoving images stored in recording media such as digital versatile discs(DVDs), portable CD players, portable radios, tape recorders, headphonestereos, stereos, table clocks, wall clocks, cordless phone handsets,transceivers, portable wireless devices, mobile phones, car phones,portable game machines, calculators, portable information terminals,electronic notebooks, e-book readers, electronic translators, audioinput devices, video cameras, digital still cameras, toys, electricshavers, high-frequency heating appliances such as microwave ovens,electric rice cookers, electric washing machines, electric vacuumcleaners, water heaters, electric fans, hair dryers, air-conditioningsystems such as air conditioners, humidifiers, and dehumidifiers,dishwashers, dish dryers, clothes dryers, futon dryers, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, flashlights, electric power tools such aschain saws, smoke detectors, and medical equipment such as dialyzers.The examples also include industrial equipment such as guide lights,traffic lights, belt conveyors, elevators, escalators, industrialrobots, power storage systems, and power storage devices for levelingthe amount of power supply and smart grid. In addition, moving objectsdriven by an electric motor using power from a power storage device arealso included in the category of electronic appliances. Examples of themoving objects are electric vehicles (EV), hybrid electric vehicles(HEV) which include both an internal-combustion engine and a motor,plug-in hybrid electric vehicles (PHEV), tracked vehicles in whichcaterpillar tracks are substituted for wheels of these vehicles,motorized bicycles including motor-assisted bicycles, motorcycles,electric wheelchairs, golf carts, boats, ships, submarines, helicopters,aircrafts, rockets, artificial satellites, space probes, planetaryprobes, and spacecrafts.

In the above electronic appliances, the power storage device of oneembodiment of the present invention can be used as a main power sourcefor supplying enough power for almost the whole power consumption.Alternatively, in the above electronic appliances, the power storagedevice of one embodiment of the present invention can be used as anuninterruptible power source which can supply power to the electronicappliances when the supply of power from the main power source or acommercial power source is stopped. Still alternatively, in the aboveelectronic appliances, the power storage device of one embodiment of thepresent invention can be used as an auxiliary power source for supplyingpower to the electronic appliances at the same time as the power supplyfrom the main power source or a commercial power source.

FIG. 8 illustrates specific structures of the electronic appliances. InFIG. 8, a display device 600 is an example of an electronic applianceusing a power storage device 604 of one embodiment of the presentinvention. Specifically, the display device 600 corresponds to a displaydevice for TV broadcast reception and includes a housing 601, a displayportion 602, speaker portions 603, the power storage device 604, and thelike. The power storage device 604 of one embodiment of the presentinvention is provided in the housing 601. The display device 600 canreceive power from a commercial power source. Alternatively, the displaydevice 600 can use power stored in the power storage device 604. Thus,the display device 600 can be operated with the use of the power storagedevice 604 of one embodiment of the present invention as anuninterruptible power source even when power cannot be supplied from acommercial power source due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 602.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like in addition to TV broadcast reception.

In FIG. 8, an installation lighting device 610 is an example of anelectronic appliance using a power storage device 613 of one embodimentof the present invention. Specifically, the installation lighting device610 includes a housing 611, a light source 612, the power storage device613, and the like. Although FIG. 8 illustrates the case where the powerstorage device 613 is provided in a ceiling 614 on which the housing 611and the light source 612 are installed, the power storage device 613 maybe provided in the housing 611. The installation lighting device 610 canreceive power from a commercial power source. Alternatively, theinstallation lighting device 610 can use power stored in the powerstorage device 613. Thus, the installation lighting device 610 can beoperated with the use of the power storage device 613 of one embodimentof the present invention as an uninterruptible power source even whenpower cannot be supplied from a commercial power source due to powerfailure or the like.

Note that although the installation lighting device 610 provided in theceiling 614 is illustrated in FIG. 8 as an example, the power storagedevice of one embodiment of the present invention can be used as aninstallation lighting device provided in, for example, a wall 615, afloor 616, a window 617, or the like other than the ceiling 614.Alternatively, the power storage device can be used in a tabletoplighting device or the like.

As the light source 612, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and a light-emittingelement such as an LED and an organic EL element are given as examplesof the artificial light source.

In FIG. 8, an air conditioner including an indoor unit 620 and anoutdoor unit 624 is an example of an electronic appliance using a powerstorage device 623 of one embodiment of the present invention.Specifically, the indoor unit 620 includes a housing 621, an air outlet622, the power storage device 623, and the like. Although FIG. 8illustrates the case where the power storage device 623 is provided inthe indoor unit 620, the power storage device 623 may be provided in theoutdoor unit 624. Alternatively, the power storage device 623 may beprovided in both the indoor unit 620 and the outdoor unit 624. The airconditioner can receive power from a commercial power source.Alternatively, the air conditioner can use power stored in the powerstorage device 623. Particularly in the case where the power storagedevices 623 are provided in both the indoor unit 620 and the outdoorunit 624, the air conditioner can be operated with the use of the powerstorage device 623 of one embodiment of the present invention as anuninterruptible power source even when power cannot be supplied from acommercial power source due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 8 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 8, an electric refrigerator-freezer 630 is an example of anelectronic appliance using a power storage device 634 of one embodimentof the present invention. Specifically, the electricrefrigerator-freezer 630 includes a housing 631, a door for arefrigerator 632, a door for a freezer 633, the power storage device634, and the like. The power storage device 634 is provided inside thehousing 631 in FIG. 8. The electric refrigerator-freezer 630 can receivepower from a commercial power source. Alternatively, the electricrefrigerator-freezer 630 can use power stored in the power storagedevice 634. Thus, the electric refrigerator-freezer 630 can be operatedwith the use of the power storage device 634 of one embodiment of thepresent invention as an uninterruptible power source even when powercannot be supplied from a commercial power source due to power failureor the like.

Note that among the electronic appliances described above, ahigh-frequency heating apparatus such as a microwave oven and anelectronic appliance such as an electric rice cooker require high powerin a short time. The tripping of a circuit breaker of a commercial powersource in use of electronic appliances can be prevented by using thepower storage device of one embodiment of the present invention as anauxiliary power source for supplying power which cannot be suppliedenough by a commercial power source.

In addition, in a time period when electronic appliances are not used,particularly when the proportion of the amount of power which isactually used to the total amount of power which can be supplied from acommercial power source (such a proportion referred to as a usage rateof power) is low, power can be stored in the power storage device,whereby the usage rate of power can be reduced in a time period when theelectronic appliances are used. For example, in the case of the electricrefrigerator-freezer 630, power can be stored in the power storagedevice 634 in nighttime when the temperature is low and the door for arefrigerator 632 and the door for a freezer 633 are not often opened andclosed. On the other hand, in daytime when the temperature is high andthe door for a refrigerator 632 and the door for a freezer 633 arefrequently opened and closed, the power storage device 634 is used as anauxiliary power source; thus, the usage rate of power in daytime can bereduced.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

(Embodiment 6)

Next, a portable information terminal which is an example of anelectronic appliance is described with reference to FIGS. 9A to 9C.

FIGS. 9A and 9B illustrate a tablet terminal 650 that can be folded.FIG. 9A illustrates the tablet terminal 650 in the state of beingunfolded. The tablet terminal 650 includes a housing 651, a displayportion 652 a, a display portion 652 b, a switch 653 for switchingdisplay modes, a power switch 654, a switch 655 for switching topower-saving-mode, and an operation switch 656.

Part of the display portion 652 a can be a touch panel region 657 a anddata can be input when a displayed operation key 658 is touched. Notethat FIG. 9A illustrates, as an example, that half of the area of thedisplay portion 652 a has only a display function and the other half ofthe area has a touch panel function. However, the structure of thedisplay portion 652 a is not limited to this, and all the area of thedisplay portion 652 a may have a touch panel function. For example, allthe area of the display portion 652 a can display keyboard buttons andserve as a touch panel while the display portion 652 b can be used as adisplay screen.

Like the display portion 652 a, part of the display portion 652 b can bea touch panel region 657 b. When a finger, a stylus, or the like touchesthe place where a button 659 for switching to keyboard display isdisplayed in the touch panel, keyboard buttons can be displayed on thedisplay portion 652 b.

Touch input can be performed on the touch panel regions 657 a and 657 bat the same time.

The switch 653 for switching display modes can switch the displaybetween portrait mode, landscape mode, and the like, and betweenmonochrome display and color display, for example. With the switch 655for switching to power-saving mode, the luminance of display can beoptimized depending on the amount of external light at the time when thetablet terminal is in use, which is detected with an optical sensorincorporated in the tablet terminal. The tablet terminal may includeanother detection device such as a sensor for detecting orientation(e.g., a gyroscope or an acceleration sensor) in addition to the opticalsensor.

Although the display area of the display portion 652 a is the same asthat of the display portion 652 b in FIG. 9A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 652 a may be different from that of the displayportion 652 b, and further, the display quality of the display portion652 a may be different from that of the display portion 652 b. Forexample, one of them may be a display panel that can displayhigher-definition images than the other.

FIG. 9B illustrates the tablet terminal 650 in the state of beingclosed. The tablet terminal 650 includes the housing 651, a solar cell660, a charge and discharge control circuit 670, a battery 671, and aDCDC converter 672. Note that FIG. 9B illustrates an example in whichthe charge and discharge control circuit 670 includes the battery 671and the DCDC converter 672, and the battery 671 includes the powerstorage device described in any of the above embodiments.

Since the tablet terminal 650 can be folded, the housing 651 can beclosed when the tablet terminal 650 is not in use. Thus, the displayportions 652 a and 652 b can be protected, thereby providing the tabletterminal 650 with excellent endurance and excellent reliability forlong-term use.

The tablet terminal illustrated in FIGS. 9A and 9B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 660, which is attached on the surface of the tabletterminal 650, supplies power to the touch panel, the display portion, avideo signal processor, and the like. Note that the solar cell 660 ispreferably provided on one or two surfaces of the housing 651, in whichcase the battery 671 can be charged efficiently. The use of the powerstorage device of one embodiment of the present invention as the battery671 has advantages such as a reduction is size.

The structure and operation of the charge and discharge control circuit670 illustrated in FIG. 9B are described with reference to a blockdiagram in FIG. 9C. The solar cell 660, the battery 671, the DCDCconverter 672, a converter 673, switches SW1 to SW3, and the displayportion 652 are illustrated in FIG. 9C, and the battery 671, the DCDCconverter 672, the converter 673, and the switches SW1 to SW3 correspondto the charge and discharge control circuit 670 illustrated in FIG. 9B.

First, an example of the operation in the case where power is generatedby the solar cell 660 using external light is described. The voltage ofpower generated by the solar cell 660 is raised or lowered by the DCDCconverter 672 so that the power has a voltage for charging the battery671. Then, when the power from the solar cell 660 is used for theoperation of the display portion 652, the switch SWI is turned on andthe voltage of the power is raised or lowered by the converter 673 so asto be a voltage needed for the display portion 652. In addition, whendisplay on the display portion 652 is not performed, the switch SW1 maybe turned off and the switch SW2 may be turned on so that the battery671 is charged.

Here, the solar cell 660 is described as an example of a powergeneration means; however, there is no particular limitation on thepower generation means, and the battery 671 may be charged with anotherpower generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 671 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

It is needless to say that one embodiment of the present invention isnot limited to the electronic appliance illustrated in FIGS. 9A to 9C aslong as the electronic appliance is equipped with the power storagedevice described in any of the above embodiments.

(Embodiment 7)

Further, an example of the moving object which is an example of theelectronic appliance is described with reference to FIGS. 10A and 10B.

Any of the power storage device described in any of the aboveembodiments can be used as a control battery. The control battery can beexternally charged by electric power supply using a plug-in technique orcontactless power feeding. Note that in the case where the moving objectis an electric railway vehicle, the electric railway vehicle can becharged by electric power supply from an overhead cable or a conductorrail.

FIGS. 10A and 10B illustrate an example of an electric vehicle. Anelectric vehicle 680 is equipped with a battery 681. The output of thepower of the battery 681 is adjusted by a control circuit 682 and thepower is supplied to a driving device 683. The control circuit 682 iscontrolled by a processing unit 684 including a ROM, a RAM, a CPU, orthe like which is not illustrated.

The driving device 683 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 684 outputs a control signal to the control circuit 682 based oninput data such as data on operation (e.g., acceleration, deceleration,or stop) by a driver of the electric vehicle 680 or data on driving theelectric vehicle 680 (e.g., data on an upgrade or a downgrade, or dataon a load on a driving wheel). The control circuit 682 adjusts theelectric energy supplied from the battery 681 in accordance with thecontrol signal of the processing unit 684 to control the output of thedriving device 683. In the case where the AC motor is mounted, althoughnot illustrated, an inverter which converts direct current intoalternate current is also incorporated.

The battery 681 can be charged by external electric power supply using aplug-in technique. For example, the battery 681 is charged through apower plug from a commercial power source. The battery 681 can becharged by converting external power into DC constant voltage having apredetermined voltage level through a converter such as an AC-DCconverter. When the power storage device of one embodiment of thepresent invention is provided as the battery 681, capacity of thebattery 681 can be increased and improved convenience can be realized.When the battery 681 itself can be made compact and lightweight withimproved characteristics of the battery 681, the vehicle can be madelightweight, leading to an increase in fuel efficiency.

Note that it is needless to say that one embodiment of the presentinvention is not limited to the electronic appliances described above aslong as the power storage device of one embodiment of the presentinvention is included.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

EXAMPLE 1

The present invention is described specifically below with Example. Notethat the present invention is not limited to Example below.

(Evaluation of Inorganic Compound Film by XRD)

A structure of niobium oxide was evaluated by X-ray Diffraction (XRD).Here, graphite was not used. Niobium which is a gel was formed by asol-gel method, put into a crucible, and baked at 600° C. for 3 hours toform niobium oxide, and the niobium oxide was measured. The measurementresults obtained by XRD are shown in FIG. 11.

In FIG. 11, the horizontal axis represents an X-ray diffraction angleand the vertical axis represents an X-ray diffraction intensity. Peaksof the XRD spectrum represent intensity of an X-ray reflected from acrystal lattice surface. The larger the intensity of a peak of thespectrum is and the narrower the half bandwidth is, the higher thecrystallinity is. Note that the crystal lattice surface and the X-raydiffraction angle correspond to each other, and a location where thepeak of the XRD spectrum appears (a diffraction angle of 2θ) variesdepending on a crystal structure and the crystal lattice surface.

The measurement results in FIG. 11 show that the inorganic compound filmformed by the sol-gel method is Nb₂O₅ having a hexagonal crystalstructure with a space group of P63 /mmc.

Note that the sample of niobium oxide for this measurement by XRD wasbaked at 600° C., and even a sample baked at 500° C. had a similarcrystal structure. On the other hand, a sample baked at low temperaturesranging from 200° C. to 300° C. becomes amorphous fine particles.

(Formation of Particulate Graphite Having Inorganic Compound Film)

Next, a negative electrode active material having a niobium oxide filmas an inorganic film was formed. As the negative electrode activematerial, graphite produced by JFE Chemical Corporation was used. First,as described in Embodiment 2, Nb(OEt)₅ and ethyl acetoacetate serving asa stabilizing agent to which toluene was added were stirred, so that aNb(OEt)₅ toluene solution was formed. The compounding ratio of thissolution was as follows: the Nb(OEt)₅ was 3.14×10⁻⁴ mol; the ethylacetoacetate, 6.28×10⁻⁴ mol; and the toluene, 2 ml. Next, the Nb(OEt)₅toluene solution to which particulate graphite that is the negativeelectrode active material was added was stirred in a dry room. Then, thesolution was held at 50° C. in a humid environment for 3 hours so thatthe Nb(OEt)₅ in the Nb(OEt)₅ toluene solution to which the graphite wasadded was hydrolyzed and condensed. In other words, the Nb(OEt)₅ in thesolution was made to react with water in the air so that a hydrolysisreaction gradually occurs, and the Nb(OEt)₅ was condensed by adehydration reaction which sequentially occurred. In such a manner,niobium which is a gel was attached on the surface of the particulategraphite. Then, baking was performed at 500° C. in the air for 3 hours,so that the particulate graphite having an inorganic compound filmcontaining niobium oxide as a component on part of its surface wasformed.

(Observation of Inorganic Compound Film with Electron Microscope)

FIGS. 12A, 12B, 13A, and 13B are images of the particulate graphiteobserved with a scanning electron microscope (SEM). FIGS. 12A and 12Bare the SEM images of particulate graphite 700 which is not providedwith a niobium oxide film. FIG. 12A is the SEM image at 3000-foldmagnification, and FIG. 12B is the SEM image at 5000-fold magnificationof part of the SEM image in FIG. 12A. The particulate graphite 700 witha diameter of approximately 20 μm and a rough surface is observed, andsmall particles aggregated on the surface of the particulate graphite700 are observed.

FIGS. 13A and 13B are the SEM images of the particulate graphite shownin FIGS. 12A and 12B part of a surface of which is provided with aniobium oxide film by the sol-gel method. Regions in a dark gray colorin the SEM images of FIGS. 13A and 13B represent particulate graphite701. On the other hand, white spot regions on a surface of theparticulate graphite 701 are portions where niobium oxide films 702 areformed. As described above, from difference in contrast, regions wherethe niobium oxide films are formed and regions where the niobium filmsare not formed can be observed in the SEM images. It is observed thatthe niobium oxide films do not completely cover the surface of theparticulate graphite 701, but the surface is partly covered.

Next, a cross section of particulate graphite having a niobium oxidefilm on part of its surface by the sol-gel method in such a manner wasobserved with a transmission electron microscope (TEM). FIGS. 14, 15A,and 15B show TEM images.

FIG. 14 is the TEM image of cross sections of particles of particulategraphite 703. The particulate graphite 703 having a niobium oxide filmon part of its surface was provided over a silicon wafer, and wascovered with a carbon film 704 and a tungsten film 706 for processingand observation of the sample.

A layered structure inside the particulate graphite 703 can be observed.The niobium oxide film can be observed in a black color in places on thesurface of the particulate graphite 703, that is, an edge portion in thecross section of the particulate graphite 703.

FIG. 15A is the enlarged TEM image of the dashed frame in FIG. 14. Inthe TEM image, the niobium oxide film is observed in a black color onthe surface of the particulate graphite 703. In addition, FIG. 15B isthe enlarged TEM image of the dashed frame in FIG. 15A. A niobium oxidefilm 705 formed on the surface of the particulate graphite 703 is formedalong the surface of the particulate graphite 703. Since the samplesliced for the observation with TEM is thick in the depth direction inFIG. 15B and the graphite that is the observation object is particles,the thickness of the niobium oxide film 705 is difficult to measure.However, the observation result shows that the niobium oxide film 705has a thickness of around 10 nm to 20 nm.

From the observation results obtained with the electron microscopes suchas SEM and TEM, the niobium oxide film can be formed on the surface ofthe particulate graphite by the sol-gel method. It is found that theniobium oxide film formed by the sol-gel method is an extremely thininorganic compound film with a thickness around 10 nm to 20 nm. Further,it is found that the niobium oxide film does not entirely cover thesurface of the particulate graphite, but partly covers the surface.

(CV Measurement 1)

Next, whether or not the inorganic compound film of one embodiment ofthe present invention inserts and extracts lithium ions was verified bycyclic voltammetry (CV).

A three-electrode cell was used in the CV measurement. An activematerial layer including particulate graphite having a niobium oxidefilm on part of its surface was used as a working electrode; metalliclithium, a reference electrode and a counter electrode; and anelectrolyte solution, 1 M lithium hexafluorophosphate (LiPF₆) dissolvedin a mixed solution of an ethylene carbonate (EC) solution (1 mol/L) anddiethyl carbonate (DEC) (volume ratio of 1:1). The measurement wasperformed at a scanning speed of 0.2 mV/sec in a scan range from 0 V to2.5 V (vs. Li/Li⁺) for 3 cycles.

The cyclic voltammograms of the CV measurement results are shown inFIGS. 16A and 16B. FIG. 16A shows the measurement results of the 3cycles in a scan range from 0 V to 2 V. FIG. 16B focuses on a potentialaround 1.5 V to 2 V in the first cycle. At the time of the insertion oflithium ions, a change in a current value can be seen inside a portionsurrounded by the dashed line on the bottom of the graph. This showsthat the niobium oxide film provided on part of the surfaces of thegraphite particles reacted with lithium in the first insertion oflithium ions.

For comparison, graphite similar to the above was not provided with aniobium oxide film on its surface, and measured by CV under the sameconditions. The measurement results are shown in FIGS. 17A and 17B. Asshown in FIG. 17B, a big change is not seen in a current value around1.5 V to 2 V. Therefore, FIG. 17B indicates that the change in thecurrent value in FIG. 16B shows a reaction between lithium and theniobium oxide film.

(TEM Observation)

Next, a negative electrode in which particulate graphite having aniobium oxide film formed by the sol-gel method as described above as aninorganic compound film was used as a negative electrode active materialwas formed. A half cell including part of the negative electrode, anelectrolyte solution, and a reference electrode was fabricated and then,the negative electrode was observed with TEM. FIG. 18 shows a TEM image.

First, a method for forming the negative electrode is described.

Slurry was formed in which the particulate graphite having the niobiumoxide formed by the sol-gel method in the above-described manner as aninorganic compound film and PVdF that is a binder were mixed in a ratioof 9:1. At this time, the proportion of the niobium oxide in thegraphite was 0.5 wt %, and NMP was used as a solvent of the slurry.

Next, copper foil was used as a negative electrode current collector.The slurry including the graphite was applied onto the negativeelectrode current collector and dried at 70° C., and then dryingtreatment was performed at 170° C. in a vacuum atmosphere for 10 hours.

A lithium electrode was used as a counter electrode. A 1 M lithiumhexafluorophosphate (LiPF₆) solution (in which a mixed solution ofethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio of1:1) was used as a solvent) was used as the electrolyte solution.

Next, charge and discharge were performed at a rate of 0.2 C (it takes 5hours for charge), constant current, and voltages ranging from 2 V to 4V. Charge and discharge were performed once in one cycle. Through theabove steps, a film was formed on the negative electrode active materialand the inorganic compound film, whereby the negative electrode activematerial layer was formed. In addition, the negative electrode includingthe negative electrode current collector and the negative electrodeactive material layer was formed. FIG. 18 shows the TEM image of thenegative electrode obtained by disassembling the battery charged anddischarged as described above. Note that Region 1 and Region 2 areprotective films for the TEM observation, and are a carbon film and a Ptfilm, respectively.

Next, energy dispersive X-ray spectrometry (EDX) analysis was performedon Regions 3 to 6 shown in FIG. 18. Results of EDX analysis of Regions3, 4, and 5 are shown in FIGS. 19A, 19B, and 19C, respectively. In theEDX analysis, the sample was thinned by an FIB method (μ-samplingmethod) using Ga ions. In addition, the sample was fixed to copper mesh.For these reasons, copper or gallium is measured in FIGS. 19A to 19C.Further, trace amounts of silicon is also detected as an impurity due tothe irradiation with an electron beam at the time of the EDX analysis.

As shown in FIGS. 19A to 19C, Region 3 contains carbon, oxygen, andfluorine; Region 4 contains niobium, carbon, oxygen, phosphorus, andfluorine; and Region 5 contains carbon as a main component. Note thatalthough not shown in FIGS. 19A to 19C, Region 6 contains carbon as amain component.

FIG. 18 and FIGS. 19A to 19C show that Region 5 is a graphite particle,and Region 4 formed on a surface of the graphite particle is a10-nm-thick inorganic compound film formed of niobium oxide. Inaddition, Region 3 formed on a surface of the inorganic compound film isa film which corresponds to a reductive decomposition product of areductive decomposition reaction of the electrolyte solution or aproduct of a reaction between the reductive decomposition product andthe electrolyte solution. Note that the carbon and the fluorine aredetected in Region 4 because of the influence of Region 3 in the EDXanalysis.

Through the above steps, the negative electrode active material layerwhich includes the negative electrode active material having theinorganic compound film on part of its surface and the film in contactwith an exposed portion of the negative electrode active material andthe inorganic compound film can be formed.

(Evaluation of Cycle Performance)

Next, a negative electrode in which particulate graphite having aniobium oxide film formed by the sol-gel method in the above-describedmanner as an inorganic compound film was used as a negative electrodeactive material was formed. The negative electrode, an electrolytesolution, and a positive electrode were assembled as a full cell. Then,the full cell was charged and discharged once to manufacture a secondarybattery. Then, the cycle performance of the secondary battery wasmeasured.

The performance was measured using coin cells. As the electrolytesolution, a 1 M lithium hexafluorophosphate (LiPF₆) solution was used.Note that as a solvent of the electrolyte solution, a mixed solvent inwhich ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed ata volume ratio of 1:1 was used. As a separator, polypropylene (PP) wasused. First charge and discharge were performed at a rate of 0.2 C (ittakes 5 hours for charge), and second charge and discharge wereperformed at a rate of 1 C (it takes 1 hour for charge). In the 200thcycle, charge and discharge were performed at a rate of 0.2 C (it takes5 hours for charge) to measure discharge capacity. Further, all chargesand discharges were performed at constant current, voltages ranging from2 V to 4 V, and an environment temperature of 60° C. Under suchconditions, the measurement was performed.

The cycle performance of each of a negative electrode includingparticulate graphite, which has a niobium film as an inorganic compoundfilm of part of its surface, as a negative electrode active material anda negative electrode including particulate graphite, which does not havean inorganic compound film, as a negative electrode active material wasevaluated. Moreover, as the graphite having the niobium oxide film, theone in which the weight ratio of niobium oxide was 0.5 wt % to thegraphite was formed.

The measurement results of the cycle performance are shown in FIG. 20.The horizontal axis represents the number of cycles (times) and thevertical axis represents discharge capacity (mAh/g) of the secondarybatteries. The number of measured samples (n) of the electrode which has0.5 wt % of niobium oxide and the electrode which does not have aniobium oxide film were two (n=2) and one (n=1), respectively. In FIG.20, curves 810 a and 810 b show the cycle performance of the electrodewhich has 0.5 wt % of niobium oxide and a curve 820 shows the cycleperformance of the electrode which does not have the niobium oxide film.

As a result of the measurement, as shown by the curve 820, in the caseof the secondary battery including the particulate graphite, which doesnot have the inorganic film containing a niobium oxide film as acomponent, as the negative electrode active material, the dischargecapacity decreases as the number of cycles increases. That is,deterioration is significant.

In contrast, as shown by the curves 810 a and 810 b, in the secondarybattery including the particulate graphite, which has the niobium oxidefilm as the inorganic compound film, as a negative electrode activematerial, although the discharge capacity tends to decrease, thecapacity is not greatly reduced, which is unlike in the secondarybattery including the particulate graphite, which does not have theinorganic compound film. Thus, it is found that deterioration issufficiently suppressed. The deterioration is particularly suppressed atan environment temperature of 60° C. Consequently, it is possible toincrease the cycle performance.

(Evaluation)

As described above, the decomposition reaction of the electrolytesolution and the like caused as a side reaction of charge and dischargeis minimized in repeated charge and discharge of a lithium ion battery,resulting in an improvement in the cycle performance of the lithium ionbattery.

This application is based on Japanese Patent Application serial No.2012-126355 filed with Japan Patent Office on Jun. 1, 2012 and JapanesePatent Application serial No. 2013-047833 filed with Japan Patent Officeon Mar. 11, 2013, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A power storage device comprising: a positiveelectrode; a negative electrode; and an electrolyte, wherein thenegative electrode comprises: a current collector; an active materiallayer over the current collector, the active material layer comprising afirst particle of an active material, and a first film comprising aninner surface on a first region of the first particle; and a second filmcovering the first particle and the first film, wherein the firstparticle comprises silicon, wherein the first film comprises an oxide ofany one of aluminum, titanium, niobium, vanadium, tantalum, tungsten,zirconium, molybdenum, hafnium, chromium, and silicon, wherein thesecond film comprises an organic material and an inorganic material,wherein the second film is in contact with a second region of the firstparticle, and wherein the second film is in contact with an outersurface of the first film.
 2. The power storage device according toclaim 1, wherein the first particle has an average diameter of more thanor equal to 6 μm and less than or equal to 30 μm.
 3. The power storagedevice according to claim 1, wherein the first film has a thickness ofmore than or equal to 5 nm and less than or equal to 50 nm.
 4. The powerstorage device according to claim 1, wherein the inorganic material isone of a fluoride, a carbonate, an oxide, and a hydroxide of a materialselected from lithium, sodium, potassium, calcium, strontium, barium,beryllium, and magnesium.
 5. The power storage device according to claim1, wherein the inorganic material is one of lithium fluoride, lithiumcarbonate, lithium oxide, and lithium hydroxide.
 6. The power storagedevice according to claim 1, wherein the active material layer furthercomprises a second particle of the active material, and a third filmcomprising an inner surface on a first region of the second particle,wherein the second particle comprises silicon, wherein the third filmcomprises an oxide of the one of aluminum, titanium, niobium, vanadium,tantalum, tungsten, zirconium, molybdenum, hafnium, chromium, andsilicon, wherein the third film is in contact with a second region ofthe second particle, wherein the second film is further in contact withan outer surface of the third film, and wherein the first particle is incontact with the third film and the second particle.
 7. The powerstorage device according to claim 1, wherein the first film is formedfrom the oxide.
 8. A power storage device comprising: a positiveelectrode; a negative electrode; and an electrolyte, wherein thenegative electrode comprises: a current collector; an active materiallayer over the current collector, the active material layer comprising afirst particle of an active material, and a first film comprising aninner surface on a first region of the first particle; and a second filmcovering the first particle and the first film, wherein the firstparticle comprises silicon, wherein the first film comprises aluminumand oxygen, wherein the second film comprises an organic material and aninorganic material, wherein the second film is in contact with a secondregion of the first particle, and wherein the second film is in contactwith an outer surface of the first film.
 9. The power storage deviceaccording to claim 8, wherein the first particle has an average diameterof more than or equal to 6 μm and less than or equal to 30 μm.
 10. Thepower storage device according to claim 8, wherein the first film has athickness of more than or equal to 5 nm and less than or equal to 50 nm.11. The power storage device according to claim 8, wherein the inorganicmaterial is one of a fluoride, a carbonate, an oxide, and a hydroxide ofa material selected from lithium, sodium, potassium, calcium, strontium,barium, beryllium, and magnesium.
 12. The power storage device accordingto claim 8, wherein the inorganic material is one of lithium fluoride,lithium carbonate, lithium oxide, and lithium hydroxide.
 13. The powerstorage device according to claim 8, wherein the active material layerfurther comprises a second particle of the active material, and a thirdfilm comprising an inner surface on a first region of the secondparticle, wherein the second particle comprises silicon, wherein thethird film comprises aluminum and oxygen, wherein the third film is incontact with a second region of the second particle, wherein the secondfilm is further in contact with an outer surface of the third film, andwherein the first particle is in contact with the third film and thesecond particle.
 14. The power storage device according to claim 8,wherein the first particle is formed from silicon, and wherein the firstfilm is formed from aluminum and oxygen.
 15. A power storage devicecomprising: a positive electrode; a negative electrode; and anelectrolyte, wherein the negative electrode comprises: a currentcollector; an active material layer over the current collector, theactive material layer comprising a first particle of an active material,and a first film comprising an inner surface on a first region of thefirst particle; and a second film covering the first particle and thefirst film, wherein the first particle comprises silicon and oxygen,wherein the first film comprises aluminum and oxygen, wherein the secondfilm comprises an organic material and an inorganic material, whereinthe second film is in contact with a second region of the firstparticle, and wherein the second film is in contact with an outersurface of the first film.
 16. The power storage device according toclaim 15, wherein the first particle has an average diameter of morethan or equal to 6 μm and less than or equal to 30 μm.
 17. The powerstorage device according to claim 15, wherein the first film has athickness of more than or equal to 5 nm and less than or equal to 50 nm.18. The power storage device according to claim 15, wherein theinorganic material is one of a fluoride, a carbonate, an oxide, and ahydroxide of a material selected from lithium, sodium, potassium,calcium, strontium, barium, beryllium, and magnesium.
 19. The powerstorage device according to claim 15, wherein the inorganic material isone of lithium fluoride, lithium carbonate, lithium oxide, and lithiumhydroxide.
 20. The power storage device according to claim 15, whereinthe active material layer further comprises a second particle of theactive material, and a third film comprising an inner surface on a firstregion of the second particle, wherein the second particle comprisessilicon and oxygen, wherein the third film comprises aluminum andoxygen, wherein the third film is in contact with a second region of thesecond particle, wherein the second film is further in contact with anouter surface of the third film, and wherein the first particle is incontact with the third film and the second particle.
 21. The powerstorage device according to claim 15, wherein the first particle isformed from silicon and oxygen, and wherein the first film is formedfrom aluminum and oxygen.
 22. A power storage device comprising: apositive electrode; a negative electrode; and an electrolyte, whereinthe negative electrode comprises: a current collector; an activematerial layer over the current collector, the active material layercomprising a first particle of an active material, and a first filmcomprising an inner surface on a first region of the first particle; anda second film covering the first particle and the first film, whereinthe first particle comprises silicon and oxygen, wherein the first filmcomprises titanium and oxygen, wherein the second film comprises anorganic material and an inorganic material, wherein the second film isin contact with a second region of the first particle, and wherein thesecond film is in contact with an outer surface of the first film. 23.The power storage device according to claim 22, wherein the firstparticle has an average diameter of more than or equal to 6 μm and lessthan or equal to 30 μm.
 24. The power storage device according to claim22, wherein the first film has a thickness of more than or equal to 5 nmand less than or equal to 50 nm.
 25. The power storage device accordingto claim 22, wherein the inorganic material is one of a fluoride, acarbonate, an oxide, and a hydroxide of a material selected fromlithium, sodium, potassium, calcium, strontium, barium, beryllium, andmagnesium.
 26. The power storage device according to claim 22, whereinthe inorganic material is one of lithium fluoride, lithium carbonate,lithium oxide, and lithium hydroxide.
 27. The power storage deviceaccording to claim 22, wherein the active material layer furthercomprises a second particle of the active material, and a third filmcomprising an inner surface on a first region of the second particle,wherein the second particle comprises silicon and oxygen, wherein thethird film comprises titanium and oxygen, wherein the third film is incontact with a second region of the second particle, wherein the secondfilm is further in contact with an outer surface of the third film, andwherein the first particle is in contact with the third film and thesecond particle.
 28. The power storage device according to claim 22,wherein the first particle is formed from silicon and oxygen, andwherein the first film is formed from titanium and oxygen.